Project Summary About 1 in 8 American women will develop invasive breast cancer over the course of her lifetime. It is the most common malignancy in American women, accounting for 30% of new cancer cases. Radiation therapy is a cornerstone of breast cancer treatment, and can improve local disease control and prolong life of breast cancer patients. Over the past decade, post-lumpectomy radiotherapy has evolved and a short version of radiotherapy called hypofractionated whole breast irradiation (HF-WBI) was introduced. HF-WBI is usually delivered over 3-4 weeks, as compare to 5-7 weeks in conventional fractionation, and can save 21 days of patient time on average. Several randomized trials have showed that HF-WBI is equivalent in local recurrence and disease-free survival to conventional fractionation, but it adoption in the U.S. remained low (<20%). HF- WBI represents an exemplary type of medical care with demonstrated effectiveness, promise of economic efficiency, but low adoption rates, and thus investigation of barriers of HF-WBI slow adoption may be relevant for understanding potentially modifiable factors of clinically effective and cost treatments of other diseases. In this proposal, we plan to investigate financial factors or incentives that are related to use of HF-WBI using a large insurance claims database, which covers claims for over 40 million people. The study has three specific aims: 1) To examine the variation of HF-WBI use relative to CF-WBI across health care providers and communities in the U.S.; 2) To investigate the influence of patient characteristics, insurance policies and financial burden to patients on HF-WBI use; and 3) To determine whether physician reimbursement structure affects HF-WBI use. Approximately 67,000 breast cancer patients diagnosed between 2008 and 2013 will be included in the analysis. We will use cross-classified multilevel models to take into account issues of clustering of patients within hospitals and within communities and to decompose the variance in HF-WBI use. To test the hypothesis that financial reasons are the driving force of the slow adoption of HF-WBI, we will examine the influence of type of insurance plans, out-of-pocket cost, and physician payment structure on HF-WBI use. HF- WBI has shown to be less costly than conventional radiotherapy, so the study has a great potential to identify strategies to increase the adoption of new treatment method and reduce the cost of medical care.